Method and apparatus for tuning waveguides



May 28, 1968 R. W. SPIKULA METHOD AND APPARATS FOR TUNING WAVEGUIDES'Filed oct. u, 1965 INVENTOR ATTORNEY United States Patent O 3,386,654METHOD AND APPARATUS FOR TUNING WAVEGUIDES Russell W. Spiltula,Winston-Salem, N.C., assignor to Western Electric Company, Incorporated,New York,

NY., a corporation of New York Filed Oct. 11, 1965, Ser. No. 494,314 14Claims. (Cl. S33-21) ABSTRACT OF THE DISCLOSURE One, or a plurality ofparamagnetic slugs are inserted and magnetically positioned inside awaveguide to tune the waveguide by attenuating undesired reections andreducing the standing wave ratio. The magnetic positioning of the sluginside the waveguide represents the optimum locations for discretecompensating depressions without repetitive drilling and tapping of thewaveguide. Alternatively, a pair of crescent-shaped paramagnetic slugsare placed and magnetically positioned in a circular waveguide to alterthe polarization of a polarized wave being propagated in the waveguide.

This invention relates to a method and apparatus for tuning waveguidesand more particularly to a method and apparatus for tuning rectangularand circular waveguides using discrete compensating elements positionedwithin the waveguides by external magnetic means.

Microwave energy is transmitted through a waveguide in the form ofelectromagnetic waves. The boundary conditions imposed by the smooth,conducting surfaces of the waveguide act to constrain theelectromagnetic waves Within the confines of the waveguide. Anyirregularity in the smoothness of the conducting surfaces will cause aportion of the incident electromagnetic wave to be reiiected backtowards the source resulting in standing waves along the waveguide and adiminution of the power received at the terminating load. Similarly, anyelement connected to the waveguide will also cause reflections if itpresents an irregularity or discontinuity to the incident wave. Suchelements include the load device itself, as well as directionalcouplers, filters, attenuators, etc., connected to the waveguide. Asharp, or in some cases, a gradual bend in the waveguide will also tendto produce undesirable reflections.

In many cases it is possible to tune out the effects of theseirregularities by the use of tuning slugs which are inserted into thewaveguide. A tuning slug usually consists of a threaded yscrewprojecting into the waveguide, parallel to the electric eld in thewaveguide. The energized slug acts as a small antenna and is excited bythe electric iield in the waveguide. The energized slug radiates asignal which cancels out part of the reilections caused by the waveguideor component irregularities. Frequently, two `or three slugs are usedsimultaneously, spaced apart 1A; wavelength or 2%; wavelengthrespectively. The phase of the reflected signal may be altered byincreasing or decreasing the length of the protrusion into thewaveguide. The microwave slug tuner is analogous to the halfwave stubused to tune coaxial transmission lines.

ln development Work on prototype waveguide components, the determinationof the optimum location for the tuning slugs is accomplished by somewhatof a trial and error technique. An exact mathematical solution is3,386,054 Patented May 28, 1968 ICC generally impossible due to thcomplexity of the equations involved. The trial and error methodrequires the repeated drilling and tapping of holes to receive theslugs, or, alternatively, the use of a slotted waveguide. The use of aslotted waveguide is undesirable, as the slot is centrally located andcannot be moved, and, further, the slot may affect the transmissioncharacteristics of the device. Once the optimum location and depth ofthe slugs has been established for the prototype device, permanentslugs, dents, or dimples, are fabricated in the production version ofthe device at the locations previously predetermined by the trial anderror technique. It is obvious that the repeated drilling and tapping ofthe prototype device to locate the best position for fabricating thetuning dents in the finished product is both time consuming andexpensive.

Accordingly, it is an object of this invention to provide a new andimproved method and apparatus for tuning a waveguide.

Another `object of the invention is to provide a new and improved methodand apparatus for tuning a waveguide where no physical connection ismade between a tuning element on the inside of the waveguide and adevice for positioning the element from the outside..

Another object of the invention is to provide a method and apparatus formagnetically positioning shaped slugs within a waveguide to alter thepolarization Iof microwaves traveling down the waveguide.

Yet another object of the invention is vto provide a new and improvedmethod and apparatus for magnetically positioning one or more slugs tolocate the optimum position for a discrete compensating depression in aprototype waveguide without repetitive drilling and tapping of thewaveguide.

These and other objects are accomplished in the invention bymagnetically positioning one, or a plurality, of paramagnetic slugsinserted inside the waveguide to attenuate undesired reflections andreduce the standing wave ratio; or, to change the polarization ofmicrowave energy being impressed through the waveguide. The waveguide isterminated at one end and energized at the other.

In one embodiment of the invention, a suitable indicating device isconnected at a convenient point on the waveguide, and by means of aplurality of magnets the paramagnetic slugs are positioned within thewaveguide until the indicating device registers the minimum number ofreflections in the waveguide. The positions of the external magnets arethen marked and the production versions of the waveguide manufacturedwith tuning dents at the marked locations.

In another embodiment of the invention for use with circular waveguides,a pair of crescent-shaped, paramagnetic slugs are placed within thewaveguide and magnetically moved relative to each other and to areference axis to correct any change in polarization which may haveoccurred during transmission of the signal down the waveguide.

Other objects and advantages of the present invention will be apparentfrom the following detailed description when considered in conjunctionwith the accompanying drawings, wherein:

FIG. 1 is a perspective view of a rectangular waveguide depicting atuning slug that is selectively positioned by a magnet in accordancewith the principles of the invention;

FIG. 2 is a cross-sectional view of an alternative embodiment of theinvention wherein a tuning slug is positioned by an electromagnet;

FIG. 3 is a cross-sectional view of a further alternative embodiment ofthe invention wherein a circular waveguide has internal crescent-shapedtuning slugs that are magnetically positioned to change the polarizationof a polarized microwave that is being impressed through the waveguide;

FIG. 4 is a perspective view of the crescent-shaped tuning slugs shownin FIG. 3;

rFIG. 5 is a di-agram illustrating various types of polarization foundin rectangular and circular waveguides; and

FIG. 6 is a diagram of a typical test arrangement connected inaccordance with the principles of the invention to locate the optimumposition for tuning rectangular and circular waveguides using discretecompensating elements.

Referring now to FIG. 1 of the drawings, there is shown a hollow,rectangular waveguide 11 having an outer surface 12 and an inner surface13. A slug 14 of paramagnetic material is inserted within the waveguide11 and a position-adjusting device 15 located to move a bar magnet I6 incontact with the outer surface 12 of the waveguide 11 to attract theslug 14 and hold it firmly in contact with the inner surface 13. In thealternative, the device 15 may be dispensed with and the bar magnet maybe then manual- 1y manipulated. The magnet 16 may be of any suitablesize and strength provided that su'icient tiux passes through the Wallof the waveguide 11 to attract and retain slug 14 against the innerusrface 13. Typically, the wall of the waveguide 11 may `be 1/16 of aninch thick, and a permanent magnet manufactured from an aluminum,nickel, cobalt alloy with a residual induction of 10,000 gausses may beused. Obviously, the use of a thicker wall in waveguide 11 will requirea corresponding increase in the ystrength of magnet 16. The dimensionsof slug 14 will also vary with the size of the waveguide, but are notcritical. The thickness of the slug y14 will, however, atect the phaseof the signal radiated by the slug.

Among the purposes of the invention is to locate the optimum positionfor fabricating tuning bumps in the production version of the waveguide.Once an approx- 'imate position has lbeen located by means of the slugand the positon marked, a yconventional screw-in tuning slug may beinserted into the waveguide at the marked location for precision tuning.This will establish the depth of the desired tuning bumps. However, ifthe apparatus of this invention is used to permanently tune a productionwaveguide, and not a prototype, then it will ybe necessary to haveseveral slugs of different thicknesses and substitute them one at a timeuntil the best tuning is obtained. Typically, the slugs may be 1/2 to3%; inch long and 3A@ to 1A inch thick. Moving the magnet 16 along oracross the outer surface 12 of waveguide l11 will cause the slug 14 tomove correspondingly along or across the inner surface `13 to tune thewaveguide.

T he inner walls of a waveguide are generally extremely smooth and thecoeiiicient of friction `between the slug 14 and the inner surface '13is small. However, in some applioations it is necessary to coat Itheslug with a thin layer of friction-reducing plastic such aspolytetrauoroethylene to reduce the coeicient of friction and permit theslug 1'4 to be positioned within the waveguide 11 with greater ease.Without this friction-reducing coating, there would be a tendency forthe slug 14 to jump or walk along the inner surface 13 with the possibleresult that the exact optimum location might be missed. A similarcoating might Ibe lapplied to the contacting surface `of magnet 16 ifrequired.

It should be emphasized that, provided slug 14 is made fromelectroconductive, paramagnetic material, neither the characteristics ofslug 14, nor the insulating characteristics of any friction-reducingcoating used, will have any affect on the radiation characteristics ofthe slug 14 and hence its tuning behavior.

FIG. 2 shows an alternative embodiment wherein an electromagnet 17 isused in place of the permanent magnet 16 of FIG. 1. The electromagnet 17consists of a soft iron core '18- about Iwhich is wound a coil 19. Thecoil 19 is connected :by leads 2'1-21 to a suitable source of D.C. power22. A switch 23 is connected in series with one of the leads 21 and whenclosed, permits current to iiow from power supply 22 to the coil 19,energizing the electromagnet 17 which attracts the slug 4, as previouslydescribed.

The choice of permanent magnet 15 or electromagnet 17 in no wayiniuences the tuning effect of slug 14. For example, in applicationswhere high iiux density is required, it may be more economical to usethe electromagnet 17 rather than the bar magnet 16, which may be quitebulky.

There is illustrated in FIGS. 3 and 4 a further alternative embodimentwherein a pair of crescent-shaped slugs 214-24 are inserted into ahollow, circular waveguide 26 and retained in position by a pair ofhorseshoe-shaped, permanent magnets 29-29- Electromagnets may besubstituted for the permanent magnets 29-29 if desired. The outerarcuate surfaces 27-27 of slugs 24j-24 should preferably have the sameradius of curvature as the inner surface '31 of circular waveguide 26and may be coated with a thin layer of friction-reducing plastic ifneeded.

Referring to FIG. 5, the various types of polarization found inmicrowaves and other radio frequency transmissions, are illustrated.FIG. 5a shows linear polarization in which the component of the electricfield intensity in the direction of the reference Y axis, Ey, is in timephase with the component of the electric eld intensity in the directionof the Z axis, EZ. This is the most common type of polarization, and isthe type of interest with respect to rectangular waveguides. The term,vertical and horizontal polarization, is sometimes used to describe aparticular type of linear polarization, and means simply that theresultant electrical field vector El. obtained by adding the vectors Ezand Ey lies in the horizontal or vertical direction.

In FIG. 5a it will `be seen that the resultant vector Er is at a45-degree angle, and the wave is neither horizontally nor verticallypolarized. With respect to rectangular waveguides, regardless of theactual orientation of the waveguide, it is conventional to definevertical polarization as occurring when the electrical eld vector isparallel to the shorter dimension of the waveguide.

Due to the nonsymmetrical structure of the rectangular waveguide and theboundary conditions imposed by the conducting walls of the waveguide,the polarization angle of a transmitted wave cannot changesubstantially. When circular waveguides are used, however, due to theprefect symmetry of the waveguide it is quite possible that substantialchanges in the polarization may occur.

FIG. 5c illustrates a circularly polarized wave where EZ and Ey areequal in magnitude, but degrees out of phase. If in transmission throughthe circular waveguide, the magnitude or phase of either the EZ or Eycomponent is altered, then the circular polarization will be changed toelliptical polarization, the degree of ellipticity depending on therelative magnitude and phase of the EZ and Ey components.

FIG. 5b illustrates elliptical polarization where the magnitude of EZ isgreater than the magnitude of Ey. This is, of course, the more generalform of non-linear polarization, the perfect symmetry required toproduce circular polarization seldom occurring in practice.

Referring again to FIG. 3, increasing or decreasing the radius ofcurvature of the inner arcuate surfaces 23 28 of slugs 24-2.r willaffect the magnitude and phase of either the E7 or Ey component of thewave; thus, by proper positioning of slugs 2li- 24- within the circularwaveguide 26, a circularly polarized wave-front traveling down thewaveguide can be changed to an elliptically polarized wave-front. Inaddition, the slugs 24--24 may be used to restore the circularpolarization of a wave-front which has been distorted by passage throughequipment connected to the waveguide.

FIG. 6 shows a typical test set-up for locating the optimum positionsfor fabricating dents or dimples in the production versions of thewaveguides. Prototype waveguide 11, having a plurality of slugs 14-14inserted therein, is terminated at one end by a dummy load 32.

A plurality of permanent magnets 16--16 retain the` slugs 14-14 againstthe upper wall of waveguide 11. One end of a directional coupler 33,having a probe 34, is connected to the other end of `waveguide 11, andsweep frequency microwave generator 36 is connected to the other end ofthe directional coupler 33. The probe `34 is connected to detector 37,whose output is displayed on an oscilloscope 38. Synchronizing signalsare fed over a lead 39 from the generator 36 to the oscilloscope 38 tokeep the horizontal sweep of the oscilloscope 38 in step 4with thefrequency sweep of generator 36. The microwave voltage sampled by theprobe 34 is converted by the detector 37 and used to modulate thevertical deflection of the oscilloscope 38.

Microwave systems used for communication purposes generally require amuch broader frequency band-width than those used for radar, forexample. Typic-ally, a waveguide used for a microwave communicationssystem has a band-width of from 500 megacycles per second to 1,000megacycles per second, and the sweep frequency generator 36 must becapable of sweeping that range.

Referring again to FIG. 1, the position-adjusting device 15 comprises anonmagnetic rod 41 attached at one end to the magnet 16 and have ahandle 42 at the other. The rod passes through an aperture 43 in amovable support member 44, which rests on an outer surface 12 of thewave-guide 11. The rod 41 is freely slidable within the aperture 43,permitting the magnet 16 to be selectively positioned along the lengthof the Iwaveguide 11.

A second rod 46, with its axis perpendicular to the axis of rod 41, isconnected at one end to support 44 by flange 47. The other end of rod 46passes through an aperture 48 in a fixed block 49, which is securelyfastened to the waveguide 11. The rod 46 is terminated in a knob 51 andis freely slidable within the aperture 48, permitting the support member44 and, hence, magnet 16, to be selectively positioned across the widthof the waveguide 11.

The method of operation is as follows: The sweep frequency generator 36and oscilloscope 38 are switched on and allowed to come up to theiroperating tempera tures. The mid-band frequency and sweep frequencydeviation of generator 36 are then adjusted to correspond to the systemcharacteristics for which the waveguide is designed. Typically, themid-band frequency may be 4,000 Imegacycles per second and the deviationplus or minus 250 megacycles per second. Any irregularities in thetransmission characteristics of the waveguide 11 over the band ofinterest will cause portions of the incident wave to be reflected backtowards the source 36. These will be sensed by the probe 34 anddisplayed on oscilloscope 38. While observing the pattern onoscilloscope 38, the magnets 16-16 and, hence, slugs 14-14, are movedindividually or in combination along the upper surface of waveguide 11until the oscilloscope indicates that there are substantially noreiiections remaining. The positions of the magnets 16-16 are marked`with a suitable marking device, and tuning dents or dimplesmanufactured at those precise locations on the production runs of thewaveguide.

As previously discussed, if even greater accuracy is desired, themagnets 16-16 and slugs 14-14 may be removed and holes drilled andtapped at the marked locations to receive conventional screw-in tuningslugs and the test repeated with the depth of protrusion of the slugsbeing varied. In this manner not only the location, but

6 the depth of the tuning dents or dimples can be established.

The method and set-up used to correct for changes in polarization incircular waveguides is basically similar to that shown in FIG. 6 except,of course, arcuate slugs 24--24 and circular waveguide 26 aresubstituted yfor slugs 14--14 and waveguide 11. Detector 37 ordirectional coupler 34 should be made sensitive to the plane ofpolarization so that oscilloscope 38 will indicate when the desireddegree of correction has been obtained. Generator 36 should be capableof generating circularly polarized waves within the waveguide.

In some applications directional coupler 33 and probe 34 are not used,and the polarizationsensitive detector 37 is connected at the far end ofthe waveguide 26 in lieu of dummy load 32. Oscilloscope 38 will thusindicate the power received by the detector rather than indicate thenumber of reflections in the waveguide. l

It is to be understood that the apparatus and methods of this inventionare not restricted to use with waveguides, but may also be used fortuning and locating the optimum positions for fabricating tuning dentsin filters, couplers, oscillator tubes, E and H bends, mitre bends,directional couplers, magic tee, hybrid junctions, etc. While theapparatus has been described as being suitable for microwave waveguides,it will be obvious that various changes and modifications may be madetherein without departing from the spirit an-d scope of the invention.

What is claimed is:

1. A method of reducing the standing wave ratio of an energizedwaveguide, which comprises the following steps:

placing a paramagnetic slug on an inner surface of said waveguide;

placing a magnet on an outer surface of said waveguide opposite saidslug; and

moving said magnet and said slug along said waveguide until saidstanding wave ratio is reduced to its minimum value.

2. A method of reducing reliections from surface irregularities in anenergized waveguide, which comprises the following steps:

placing paramagnetic slugs on the inner surface of said waveguide nearsaid surface irregularities;

placing a plurality of magnets on the outer surface of said waveguideopposite said slugs; and

moving said magnets along said waveguide to move said slugs to reducethe refiections in said waveguide.

3. A method of tuning a section of waveguide along which is beingimpressed microwave energy, which cornprises:

placing a slug of paramagnetic material against one inner `wall of saidwaveguide; moving a permanent magnet -along the outer wall of waveguideto advance the paramagnetic slug; and

ymeasuring and comparing the input and output energy of said microwaveenergy while said paramagnetic slug is being advanced to obtain anindication of maximum transmission of said energy with respect to thepositioning of said slug. 4. A method of altering the polarization of anonlinearly polarized microwave impressed through a circular Waveguide,which comprises:

inserting a pair of crescent-shaped slugs within the waveguide, theouter arcuate surfaces of said slugs conforming to the inner surface ofsaid waveguide;

energizing one end of the waveguide with a source of nonlinearlypolarized microwaves;

connecting a polarization-sensitive detecting device with a visualdisplay to the other end of the waveguide; and

rotating a magnetic field of sufficient strength about said waveguide tosupport and rotate said crescent-shaped slugs about the centr-al axis ofsaid waveguide and along 4the guide wall while observing said visualdisplay of the alteration in polarization.

5. A method of determining the optimum position for fabricating discretecompensating elements in a waveguide, which comprises:

placing a plurality of paramagneti'c slugs on an inner surface of saidwaveguide;

placing a `plurality of magnets on an outer surface of said waveguideopposite said slugs;

terminating said waveguide at one end with a dummy load;

connecting a directional coupler having a probe to the other end of saidwaveguide;

connecting a detector including an oscilloscope to the probe of saiddirectional coupler;

applying a source of swept frequency microwaves to said directionalcoupler and waveguide;

moving said mangets and said slugs while observing said oscilloscopeuntil a minimum reflected signal is observed; and

marking the positions of said magnets on said waveguide for subsequentfabrication of permanent discrete compensating elements at saidpositions. 6. A method of fabricating a plurality of discretecompensating tuning dents in a production microwave component having anidentical prototype microwave component, which comprises:

placing a plurality of paramagnetic slugs on an inner surface of saidprototype microwave component;

placing a plurality of magnets on an outer surface of said prototypemicrowave component opposite said slugs;

terminating said prototype microwave component at one end with a dummyload;

connecting a directional coupler having a probe to the other end of saidprototype microwave component; connecting a detector having a visualdisplay output to the probe of said directional coupler;

applying a source of swept frequency microwaves to said directionalcouple-r and said prototype microwave component;

moving said magnets and said slugs while observing said visual displayuntil a first minimum reflected signal is observed;

marking the positions of said magnets on said prototype microwavecomponent;

removing said magnets from the outer surface of said component and saidslugs from the inner surface of said prototype microwave component;

drilling and tapping holes in the wall of said prototype microwavedevice at said marked positions;

inserting threaded screw-in tuning slugs in said tapped holes;

adjusting said screw-in tuning slugs while observing said visual displayuntil a second minimum reflected signal is observed; recording thedepths of penetration of said conventional slugs in said prototypemicrowave component; and

fabricating tuning dents in the wall of said production microwavecomponent, the location :and depths of said dents corresponding exactlyto the location and depth of protrusion of said conventional slugs insaid prototype component.

7. An apparatus for altering the polarization of an electromagnetic wavewhich comprises:

a `crescent-shaped paramagnetic radiating member excited by saidelectromagnetic wave; and

magnetic means, external to the field of said electromagnetic wave, formoving said radiating member about the center of curvature of its convexsurface within the field of said electromagnetic wave.

8. An electrical tuning device, which comprises:

a paramagnetic radiating member;

means for impressing microwave energy on said paramagnetic member toexcite said member to radiate a signal proportional to said impressedenergy; and magnetic means external to the field of said microwaveenergy for supporting and moving said radiating member within, andadjoining the boundary of, said eld.

9. Apparatus for tuning a waveguide, which comprises:

a source of microwave power for impressing microwaves through saidwaveguide;

a paramagnetic slug freely placed on an inner wall of said waveguide;

an external magnet movably mounted adjacent to an outer wall and inregister with said paramagnetic slug;

means for moving the magnet along the outer wall of the waveguide toadvance the paramagnetic slug along the inner wall of the waveguide; and

means for indicating variations in power of the microwaves impressedthrough said waveguide.

lil. A device for transmitting circularly polarized microwaves, whichcomprises:

a circular waveguide;

a pair of crescent-shaped paramagnetic slugs freely positioned withinsaid waveguide;

a thin layer of friction-reducing material on said pair of slugs;

a first magnetic device movably mounted on the outside of the waveguidefor selectively positioning a first one of said slugs with its outerarcuate crescent surface in contact with the inner periphery of saidwaveguide; and

a second magnetic device movably mounted on the outside of the waveguidefor selectively positioning a second one of said slugs with its outerarcuate crescent surface in contact with the inner periphery of saidwaveguide. 11. A device for optimizing tne transmission characteristicsof a length of circular waveguide, which comprises:

a slug of paramagnetic material contacting the inner surface of saidwaveguide and having a crescentshaped cross-section, the greater radiusof curvature of said cross-section corresponding to the radius ofcurvature of said inner surface;

a thin layer of friction-reducing material on said slug;

magnetic means, contacting the outer surface of said waveguide andmovable to any desired position along the length of said waveguide, forpositioning said slug around the circumference of said inner surface,said slug and said magnetic means defining a magnetic circuit havingsufficient flux through the wall of said waveguide to maintain said slugand said magnetic means in juxtaposition; and

means responsive to the transmission characteristics of said waveguidefor indicating when said slug has ben positioned to optimize saidtransmission characteristics.

12. An apparatus for altering the polarization of a nonlinearlypolarized wave being propagated inside a cylindrical waveguidecomprising:

a crescent-shaped radiating member inserted within the waveguide, saidmember having its convex surface contiguous with a portion of the insidesurface of a discrete waveguide section and its concave surface shapedat its extremities to form a continuous boundary with the remainder ofthe inside surface of the discrete waveguide section, for extracting andreradiating energy from the incident wave to alter the polarization ofsaid wave.

13. An apparatus for altering the polarization of a nonlinearlypolarized wave being propagated inside a cylindrical waveguidecomprising:

a crescent-shaped radiating member having a convex surface, the circleof curvature of which coincides with the circle of curvature of theinner guide wall of the waveguide, and a concave surface, the outerextremities of which coincide with the outer extremities of said convexsurface; and

means to support said crescent-shaped radiating member within thewaveguide with its convex surface in References Cited `contiguousrelationship with the inner guide wall of UNITED STATES PATENTS thewaveguide to expose said radiating member tothe ener of the incidentwave for alte 'in the olarization if; Said vs g P 5 2,917,719 12/1959Brown 333-7 14. An apparatus as defined in `claim 13 wherein: 3116617251/1965 Enger 33:533 said crescent-shaped radiating member is made ofparan. 4 magnetic material; and HERMAN KARL SAALBACH, P/mmly Examiner.said Supporting means includes means for applying a L. ALLAHUT,Assistant Examiner.

magnetic force to support said radiating member, 10

2,840,820 6/1958 Southworth.

